Realizing p-channel semiconducting polymers with good hole mobility, solution processibility, and air stability is an important step forward in the chemical manipulation of charge transport in polymeric solids and in the development of low-cost printed electronics. We report here the synthesis and full characterization of the dithienosilole- and dibenzosilole-based homopolymers, poly(4,4-di-n-hexyldithienosilole) (TS6) and poly(9,9-di-n-octyldibenzosilole) (BS8), and their mono- and bithiophene copolymers, poly(4,4-di-n-hexyldithienosilole-alt-(bi)thiophene) (TS6T1,TS6T2) and poly(9,9-di-n-octyldibenzosilole-alt-(bi)thiophene) (BS8T1,BS8T2), and examine in detail the consequences of introducing dithienosilole and dibenzosilole cores into a thiophene polymer backbone. We demonstrate air-stable thin-film transistors (TFTs) fabricated under ambient conditions having hole mobilities as large as 0.08 cm2/V·s, low turn-on voltages, and current on/off ratios > 106. Additionally, unencapsulated TFTs fabricated un...

Synthesis, characterization, and transistor response of semiconducting silole polymers with substantial hole mobility and air stability. Experiment and theory.

Energetics of molecular interfaces

The fundamental physical and chemical phenomena that can occur at interfaces between conjugated organic molecules and metal surfaces are discussed. The adsorption strength of molecular monolayers on metals covers a wide range from the weak physisorption regime to strong chemisorption, involving charge transfer and/or covalent bond formation. In many cases, molecular conformation changes can be observed, which directly impact the interface electronic structure and charge injection across the organic/metal contact. The energy level positions of organic multilayers are essentially determined by the monolayer/metal interaction, and 'flat-band' conditions prevail for thicker layers of pure organic molecules. Consequently, thermodynamic equilibrium across organic semiconductor films may not always be established.

https://iopscience.iop.org/article/10.1088/0953-8984/20/18/184008/pdf

Energy levels at interfaces between metals and conjugated organic molecules

A physics-based analytical model for the current-voltage (I-V) characteristics corresponding to the low and high resistive states in electroformed metal-insulator-metal structures with HfO2 layers is proposed. The model relies on the Landauer theory for the electron transport in mesoscopic systems. The switching phenomenon is ascribed to the modulation of the constriction's bottleneck cross-sectional area associated with atomic rearrangements within the confinement path. The extracted parameter values allow one to conclude that the length and radius of the region that controls the conduction characteristics are in the nanometer range.

Model for the Resistive Switching Effect in $ \hbox{HfO}_{2}$ MIM Structures Based on the Transmission Properties of Narrow Constrictions

Inserting molecular monolayers within metal/semiconductor interfaces provides one of the most powerful expressions of how minute chemical modifications can affect electronic devices. This topic also has direct importance for technology as it can help improve the efficiency of a variety of electronic devices such as solar cells, LEDs, sensors, and possible future bioelectronic ones. The review covers the main aspects of using chemistry to control the various aspects of interface electrostatics, such as passivation of interface states and alignment of energy levels by intrinsic molecular polarization, as well as charge rearrangement with the adjacent metal and semiconducting contacts. One of the greatest merits of molecular monolayers is their capability to form excellent thin dielectrics, yielding rich and unique current–voltage characteristics for transport across metal/molecular monolayer/semiconductor interfaces. We explain the interplay between the monolayer as tunneling barrier on the one hand, and th...

Chemical Modification of Semiconductor Surfaces for Molecular Electronics

Evaporated pentacene thin films with thicknesses from several nm to 150 nm on gold and silver substrates have been studied by ultraviolet photoelectron spectroscopy (UPS), near-edge X-ray absorption fine structure (NEXAFS), scanning tunneling microscopy (STM), and atomic force microscopy (AFM). It was found that pentacene thin-film structures, particularly their molecular orientations, are strongly influenced by the metal substrates. UPS measurements revealed a distinct change in the valence band structures of pentacene on Au compared to those on Ag, which is attributed to the different packing between adjacent molecules. Using NEXAFS, we observed 74 ± 5° and 46 ± 5° molecular tilt angles on Ag and Au, respectively, for all measured thicknesses. We propose that pentacene molecules stand up on the surface and form the “thin-film phase” structure on Ag. On Au, pentacene films grow in domains with molecules either lying flat or standing up on the substrate. Such a mixture of two crystalline phases leads to an average tilt angle of 46° for the whole film and the change in valence band structures. STM and distance−voltage (z−V) spectroscopy studies confirm the existence of two crystalline phases on Au with different conducting properties. z−V spectra on the low conducting phase clearly indicate its nature as “thin-film phase”.

Effect of molecule-substrate interaction on thin-film structures and molecular orientation of pentacene on silver and gold.

The silver∕polyparaphenylene interface is investigated using ballistic electron emission microscopy (BEEM). Multiple injection barriers and spatial nonuniformity of carrier injection over nanometer length scales are observed. No unique injection barrier is found. Physical reasons for these features are discussed. BEEM current images and the surface topography of the silver film are uncorrelated.

Ballistic emission spectroscopy and imaging of a buried metal∕organic interface

In this work, we present an investigation of the Pt and Pd-HfO2-p-Si interfaces using ballistic electron emission microscopy. The band alignment of the Pt-HfO2-p-Si structure is inferred. The potential drop in the oxide has been determined. Oscillations in the collector current with increasing bias enable estimation of the effective mass of electrons in HfO2 in the range of 0.35-0.44 m0. Stressing studies indicate modest resistance to stressing, with a threshold of 0.5 nC for damage to the base/oxide. Our work is the first successful application of the BEEM technique to metal-high K dielectric interfaces.

BEEM studies on metal high K-dielectric HfO2 interfaces

In this work, we present an investigation of the Ag–PPP (polyparaphenylene) interface using ballistic electron emission microscopy. Our work is the first successful application of the BEEM technique to metal–organic interfaces. We observe nanometre scale injection inhomogeneities. They have an electronic origin, since we find corresponding Schottky barrier variations. We also determine the transmission function of the Ag–PPP interface and find that it agrees qualitatively with the theoretical calculations for a metal–phenyl ring interface. We conclude that charge transport across inhomogeneous barriers needs to be considered to understand electronic transport across metal–organic interfaces and organic device characteristics.

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Metal–organic interfaces at the nanoscale

Based on the assumption that the contact barrier height determines the current flow in organic semiconductor-based electronic devices, charge injection at metal-organic (MO) interfaces has been extensively investigated, while space-charge conduction in organic bulk is generally overlooked. Recent theoretical modeling and simulation have pointed out that such a simplification is questionable due to the hopping nature of charge injection and hopping-related space-charge conduction. Here we show experimentally that charge transport in MO diodes is a complex interplay between injection-limited current (ILC) and space-charge-limited current (SCLC). We report the experimental observation of ILC-to-SCLC transition in Ag/pentacene/Ag diodes as a function of temperature.

N. Chandrasekhar

Papers

Effect of molecule-substrate interaction on thin-film structures and molecular orientation of pentacene on silver and gold.

Ballistic emission spectroscopy and imaging of a buried metal∕organic interface

BEEM studies on metal high K-dielectric HfO2 interfaces

Metal–organic interfaces at the nanoscale

Temperature-dependent transition from injection-limited to space-charge-limited current in metal-organic diodes